Just two notes I felt worth throwing in here. The DKs got hit with the nerf stick in 3.2 so you might want to see if they are still a good DPS type class. I can see them wearing alot of armor and having pets being good benifits still especially when they are not Supposed to be tanking.

Secondly I would put in a vote for the shaman healer. It would be a nice addition especially with the insta-drop of totems now. Might even want to weigh out the balance of two sets of totems vs a third DK DPS.

Just to close this up I am again all in theory over this, I have not gotten that far but from reading what you wrote and it being pretty much my end goal as well (No real interest in raiding but I might try out any 5mans, if they exist) it is what I have been putting thought into for the last month or so.